1. Field of the Invention
The present invention relates to an optical amplifier, and particularly relates to an optical amplifier which uses an optical fiber containing a rare earth additive.
2. Background Art
An example showing a structure of such an optical amplifier is depicted in FIG. 7. A input port 1 is connected to an optical fiber 2 containing the rare earth additive, the output portion of the optical fiber 2 is connected to a port #1 of an optical multiplexing and splitting device 3, and the port #2 of the optical multiplexing and splitting device 3 is connected to the output port 4. The port #3 of the optical multiplexing and splitting device 3 is connected with an excitation light multiplexer 5, and two excitation light sources 6A and 6B having respective light receivers 7A and 7B are connected to the excitation light multiplexer 5.
The light input from the input port 1 is amplified by the optical fiber containing a rare earth additive which is in the excitation state due to the excitation light from the excitation light sources 6A and 6B. Since the amplification factor at this type of amplification changes with the quantity of the excitation light emitted from the excitation light source, the light output fluctuates when the quantity of the excitation light fluctuates. Thus, an excitation light fixing control system is adopted, which controls the respective outputs of excitation light sources 6A and 6B by an control circuit such that light quantities received by respective light receivers 7A and 7B of the excitation light sources 6A and 6B are constant.
As shown in FIG. 7, two excitation light sources 6A and 6B are provided in order to increase the output power of the excitation light and also to improve the reliability of the system by driving another source if one source is damaged.
In the conventional structure shown in FIG. 7, 9 problems arise, as shown above, that the output level of the excitation light fluctuates when the actual input of the excitation light power into the optical fiber including the rare earth element fluctuates due to the tracking error of the excitation light sources 6A and 6B, or due to the polarization light dependence and the temperature characteristic of the excitation light multiplexer 5 or the like.
It is therefore an object of the present invention to provide an optical amplifier which is capable of maintaining the excitation light output at a constant level even if the excitation light input into the optical fiber containing the rare earth element fluctuates due to the tracking error of the excitation light sources, or due to the polarization dependence and the temperature characteristic of the excitation light multiplexer or the like.
According to the first aspect of the present invention, a light amplifier comprising: a optical fiber containing a rare earth additive; a plurality of excitation light sources for generating excitation light beams for said optical fiber; an excitation light multiplexing and splitting means for splitting an excitation light beam supplied into said optical fiber after multiplexing said plurality of excitation light beams and after passing through said optical fiber; a control means for controlling the output level of each excitation light sources in response to the excitation light beam split by said excitation light multiplexing and splitting means.
The light amplifier of the present invention has a feature that said excitation light multiplexing and splitting means is disposed at the output portion of said optical fiber, or disposed at the input portion of said optical fiber. Furthermore, this invention comprises a pair of excitation light sources and a light multiplexing means for multiplexing said pair of light beams emitted from said pair of light emitting sources . In addition, the light amplifier comprises a plurality of sets of a combination of the light emission sources and said light multipexer.
The light amplifier of the present invention has a feature that the light multiplexing and splitting device is separated into a light multiplexing portion and a light splitting portion, wherein the light multiplexing portion is disposed at the output portion of the optical fiber and the light splitting portion is disposed at the input portion of the optical fiber. The other feature is that said excitation light multiplexing and splitting device is separated into a light multiplexing portion and a light splitting portion, wherein the light multiplexing portion is disposed at the input portion of the optical fiber, and the light splitting portion is disposed at the output portion of the optical fiber. In the present invention, the pair of excitation light sources emits respective light beams having an identical wavelength and both excitation beams are multiplexed by a polarization composition such that neither polarizing planes interferes with the other. The plurality of sets of a pair of the excitation light sources emits excitation light beams having different wavelength from the others in the absorption band of the optical fiber so as not to influence each other.
The operation of the present invention is as follows. The light amplifier is constituted such that the output level of the excitation light beams is controlled in response to the level of the excitation light beams extracted as split beams after passing through the rare earth containing optical fiber. Thus, the light output can be controlled at a constant level even if the excitation light power fluctuates due to the tracking error of the excitation light sources 6A and 6B, or due to the polarization light dependence and the temperature characteristic of the light multiplexer for supplying the excitation light into the rare earth containing optical fiber.